Transparent electrical conductors are widely used in the flat-panel display industry to form electrodes that are used to electrically switch light-emitting or light-transmitting properties of a display pixel, for example, in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays.
In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square).
Transparent conductive metal oxides are well known in the display and touch-screen industries and have a number of disadvantages, including limited transparency and conductivity and a tendency to crack under mechanical or environmental stress. Typical prior-art conductive electrode materials include conductive metal oxides such as indium tin oxide (ITO) or very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. These materials are coated, for example, by sputtering or vapor deposition, and are patterned on display or touch-screen substrates, such as glass. For example, the use of transparent conductive oxides to form arrays of touch sensors on one side of a substrate is taught in U.S. Patent Publication 2011/0099805 entitled “Method of Fabricating Capacitive Touch-Screen Panel”.
Transparent conductive metal oxides are increasingly expensive and relatively costly to deposit and pattern. Moreover, the substrate materials are limited by the electrode material deposition process (e.g. sputtering) and the current-carrying capacity of such electrodes is limited, thereby limiting the amount of power that can be supplied to the pixel elements. Although thicker layers of metal oxides or metals increase conductivity, they also reduce the transparency of the electrodes.
Transparent conductive oxides (TCOs) are used in applications where materials are required to conduct electricity and transmit visible light with little absorption and reflection losses. Applications include touch panels, electrodes for LCD, OLEDs, electrochromic and electrophoretic displays, solid-state lighting, solar cells, energy conserving architectural windows, defogging aircraft and automobile windows, heat-reflecting coatings to increase light bulb efficiency, gas sensors, antistatic coatings, and wear resistant layers on glass. ITO is the most commonly used TCO and is typically made by electron beam evaporation or by sputtering. The properties of the ITO electrodes are highly dependent on the deposition conditions which affect the number of oxygen vacancies and carriers in the material as described in “Properties of tin doped indium oxide thin films prepared by magnetron sputtering by Ray Swati, R. Banerjee, N. Basu, A. K. Batabyal, and A. K. Barua in the Journal of Applied Physics 54(6), 3497 (1983).
Indium is in high demand and cost is expected to rise. Alternative materials are of great commercial interest including aluminum-doped zinc oxide (AZO), indium-gallium-doped zinc oxide (IGZO) and other examples of doped zinc oxide (ZnO).
Alumina (Al2O3) passivation has been shown to stabilize the columbic and thermal keeping properties of field effect transistors made with ZnO for example as described in “Passivation of ZnO TFTs” by D. A. Mourey, M. S. Burberry, D. A. Zhao, Y. V. Li, S. F. Nelson, L. Tutt, T. D. Pawlik, D. H. Levy, T. N. Jackson in the Journal of the Society for Information Display, vol. 18, issue 10, October 2010. It is well known in the art that relatively thick alumina layers (>100 nm) stabilize AZO films from environmental effects, as described in ALD 2013, 13 International Conference on Atomic Layer Deposition Abstracts, “Spatial ALD of transparent conductive oxides” by A. Illiberi, T. Grehi, A. Sharma, B. Cobb, G. Gelinck, P. Poodt, H. Brongersma and F. Roozeboom, and, 97(2013).
It is also well known that the atomic layer deposition (ALD) process produces high-quality, highly conformal films useful in many applications; however ALD is slower than many other deposition processes and therefore applications using ultra-thin layers (<50 nm) are of great practical interest.